Light modulators operating at fast frame rates (kilohertz or faster) are in great demand for optical data processing and adaptive optics applications as well as for color projection displays using a time sequential color scheme. Much progress has been made in the last thirty years in developing optical switches or modulators, but current devices are unsatisfactory for many applications. For instance, the majority of active fiber-optic devices used in present day systems, are based on an electromechanical modulator. In one type, the optical fibers are positioned end to end and mechanically moved in or out of line. In another type, mirrors are rotated to direct beams into or away from a receiving fiber. This can be accomplished mechanically or with piezoelectric or electrostatic drivers. These mechanical devices intrinsically lack speed and long term reliability.
Alternatively, fast (less than one microsecond) optical switches, using a solid electro-optic crystal in which birefringence can be induced by application of an electric field to the crystal, have been developed. Operation is based on rotating the plane of polarization of light with respect to the orientation of an analyzer that blocks or transmits light depending on the polarization direction. The basic arrangement works efficiently if incoming light is polarized with a particular orientation. However, randomly polarized light suffers a loss. This deficiency may be overcome by using additional elements that split incoming light into two orthogonal polarizations, passively rotating one to match the other, and combining the two into a single beam fed to the basic modulator. However, the suggested electro-optic crystals require voltages of one kV or more for operation. Accordingly, such devices are not well suited for many applications, including for telecommunication devices.
Additional modulators have been constructed using a tapered plate, a Faraday rotator or solid electro-optic crystal, and a second tapered plate. The Faraday rotator is controlled by varying the current in an external coil, which varies a magnetic field. But, the suggested electro-optic crystals require inefficient kilovolt drive voltages. Also, electrode design also effects polarization dependence and modulation efficiency.
Liquid crystals are an interesting medium for electro-optical effects due to their large optical birefringence and dielectric anisotropy. For example, it is known to utilize a variety of modes of a liquid crystal cell such as π-cells, and optically controllable birefringent (OCB) cells. Unfortunately, such liquid crystal based light modulators have relatively slow response times and cannot be operated typically faster than video rates (30-80 Hz). The transient nematic effect operating in the reflective mode has been proposed to achieve fast response times in a liquid crystal cell. Fast speed is achieved by only utilizing the surface layer of a nematic cell. The bulk of the cell remains unchanged. Utilizing only the surface produces only a low phase retardation.
To overcome the above limitations, liquid crystal devices containing polymer have been developed over the past decades. These devices can be divided in two subsystems: polymer dispersed liquid crystals (PDLC); and polymer stabilized liquid crystals (PSLC). In a PDLC device, a liquid crystal exists in the form of micro-sized droplets, which are dispersed in a polymer matrix. The concentration of the polymer is comparable to that of the liquid crystal. The polymer forms a continuous medium while the liquid crystal droplets are isolated from one another. These materials have been successfully used in displays, light shutters and switchable windows. Further, there has been suggested an idea to use stretched PDLC films for producing electrically controlled polarizers. The operating principle of a PDLC polarizer is based on anisotropic light scattering of PDLC films resulting from unidirectionally oriented nematic droplets. The liquid crystal domains imbedded in the confined geometry of a polymer matrix are currently among the fastest known switching devices. Unfortunately, such systems have low filling factors and liquid crystal domain size. Moreover, these devices are only known to provide light amplitude modulation, but not light phase modulation, which is critical for beam steering applications.
To speed up the switching further, there have also been attempts to change the shape of the droplets from the spherical to ellipsoidal. This idea was also realized to produce electrically controlled polarizers. The operating principle of a PDLC polarizer is based on anisotropic light scattering of PDLC films resulting from unidirectionally oriented nematic droplets. Unfortunately, such systems have low filling factors and these devices are only known to provide light amplitude modulation, but not light phase modulation which is critical for various applications. Further, stretched PDLC devices, even at high shearing deformations, never become fully transparent.
In a PSLC device, the polymer concentration is usually less than 10 wt %. The polymer network formed in such a liquid crystal cell is either anisotropic and mimics the structure of the liquid crystal or is randomly aligned. Because of the relatively low polymer content, the size of the liquid crystal domains are relatively large (>λ), and therefore, the switching times are not short enough to use in fast switching devices. Higher polymer content produces more dense polymer networks that result in significant light scattering in the cells.
In many cases, a desirable mode of operation is to switch large phase retardation within a short period of time. The maximum retardation shift ΔLmax=(ne-no)d is a linear function of the cell thickness d, while the switching time varies as d2. The no and ne are the ordinary and extraordinary refractive indices, respectively. When the field is switched off, a typical liquid crystal cell with ne-no≈0.2 and d=5 μm switches ΔLmax=1 μm within τoff=γ1d2/π2K˜25 ms, where γ1˜0.1 kg m−1 s−1 and K˜10−11 N are the characteristic rotation viscosity and elastic constant, respectively.
Based upon the foregoing, it is evident that there is still a need in the art for a liquid crystal device which has improved switching times, which can provide maximum phase retardation and still provide minimal scattering of light in the various modes.